|RUST: The other day I was told by a man who does
body and fender work on cars that "car cancer"
(rust) has to be completely removed and in the
really bad places cut out and a new piece of
metal welded in or the rust would just continue
to destroy a car. I thought that rust was the
result of iron and oxygen reacting creating iron
oxide! If I cover up the rust with paint or
"bondo" (sp?), and air can't get to it (no oxygen
so no reaction), why would it continue to get worse?
|Question Date: 1998-09-15|
You are absolutely right. Except for the fact
that paint or bondo doesn't stick well to rust so
it wouldn't seal the air out.
If you really could seal something perfectly and
exclude all oxygen, then the oxidation reaction
COULD NOT go on. However, it is really almost
impossible to seal off the metal. For one thing,
oxygen can diffuse through epoxy and paint and
hence eventually react with metal to produce the
The rate of diffusion is enhanced in the
presence of moisture. Essentially all materials
are full of tiny (nanometer- sized) cracks and
imperfections--sometimes called DEFECTS.Oxygen can
exploit these defects and other vacancies at the
You are basically correct in that rust is iron
oxide, which is a reaction between oxygen and
iron. The reaction can also be between water and
iron, or other sources of oxygen. The main reason
to remove the existing rust prior to painting,
bondo-ing, etc. is that the rust only loosely
adheres to the underlying metal, is very brittle,
and does not have much mechanical strength.
Hence, anything put over the rust will quickly
crack, debond, peel, etc., allowing oxygen and/or
water to continue forming more rust.
Your question is very apt, it does seem like
covering rust will stop its growth. However, I
must point out that rust actually requires oxygen,
iron and water as a mediator. (Without water, the
reaction is so slow, no rust would be visable for
centuries.) Oxygen dissolves readily in water, so
the key problem is to keep water (and the oxygen
it carries) away from the iron surface. This
sounds simple, but it is nearly impossible to
prevent some wetting of the surface since the
paint (or bondo epoxy) coating is porous. A good
seal can be made to bright, clean iron or steel,
but iron oxide is a weak, crumbly, and very porous
material. Worse, iron oxide forms a number of
hydrates, storing water and oxygen for further
corrosion. So, even if a seal is achieved, the
corrosion process can continue underneath it for a
time. This usually leads to loss of the seal
integrity and further corrosion. One might guess
that some chemicals can be added to the rust to
stop its growth and then the sealant can be
applied. Phosphoric acid was used in ths way for
many years, both as a rust remover (Naval jelly)
and as a
preservative. However, it is diffiuclt
to determine if the corrosion is stopped or only
coated... Thus it is often easier to simply
totally remove the rust to clean metal (which is
easy to inspect) and then rebuild the desired
shape or paint the surface.
discussion, and your own investigations of
corrosion, it might seem that iron ships are
completely silly -- yet the oldest existing
floating vessels (such as the Star of India, in
San Diego) are iron hulled or iron plated. How can
this be true? Why don't iron hulls rapidly rust
away? Hint: you might look up galvinized and
stainless steel for further insight.
post-thought, DeLorian cars made in the early
seventies from stainless steel bodies are still
rust free and are in high demand on the rebuilder
circuit. Why do you think car companies might not
want a car that lasts 30 years?
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